feat(ui): sidebar lists workspaces with their sessions, Claude Desktop style

The sidebar's project tree becomes a flat workspace list (WorkspaceTree):
one row per workspace in the stored order, each expanding to its sessions.
A session that still has a live pane opens /workspace?lane=<id>&run=<runId>,
which only attaches pane 0 to that tmux session; any other session opens
its read-only transcript. Nothing in the sidebar starts or resumes a run.

client/README.md

@@ -183,7 +183,7 @@ client/
 │   │   ├── EventDetail.tsx  # Inline hook payload viewer (used by ActivityFeed + SessionDetail)
 │   │   ├── EmptyState.tsx
 │   │   ├── Sidebar.tsx
-│   │   ├── ProjectTree.tsx  # Sidebar Projects section: workspaces grouped by repo (`GET /api/projects`), untracked worktrees + Adopt, row menu
+│   │   ├── WorkspaceTree.tsx  # Sidebar Workspaces list: one row per workspace, expanding to its sessions (live → its pane, else the transcript), row menu
 │   │   ├── StatusBar.tsx    # Fixed bottom bar: Live/Disconnected, version, running / needs-you counts, today's cost
 │   │   ├── Layout.tsx
 │   │   ├── RemoteSources.tsx  # Remote Data Sources settings panel (SSH multi-machine collection)